User GPS receiver architectures vary widely and can supply a wide variety of measurements in addition to pseudorange and continuous carrier phase, the two primary characteristic measurements from GPS. Pseudorange, xcfx81uk from Equation (1) below, is the receiver measurement of the geometric range to the satellite with degradation from satellite and receiver clock errors, the atmosphere and receiver errors. The second primary measurement of the receiver is the continuous carrier phase, xcfx86uk in Equation (2) below. Continuous carrier phase shares the same degradation factors as the pseudorange, but an additional uncertainty is added since the wavelength of the carrier is only 19 centimeters and has an integer ambiguity that is difficult to resolve in real-time.
As implied by Equations (1) and (2), a number of factors conspire to corrupt the pseudorange and carrier phase measurements for GPS. These errors are summarized below.
xcfx81uk=rukxc2x71uk+buxe2x88x92Bk+Iuk+Tuk+vukxe2x80x83xe2x80x83(1)
xcfx86uk=rukxc2x71uk+buxe2x88x92Bk+Iuk+Tuk+NukxcexL1+"xgr"ukxe2x80x83xe2x80x83(2)
where
xcfx81uk≐the pseudorange from the user receiver, u, to the kth satellite
xcfx86uk≐the continuous carrier phase from the user receiver, u, to the kth satellite
1uk≐the line-of-sight from the user receiver, u, to the kth satellite
rukxc2x71uk≐the calculated range from the user receiver, u, to the kth satellite
bu≐the user receiver clock offset from GPS time
Bk≐the kth satellite clock offset from GPS time
Iuk≐the ionospheric delay along the line-of-sight from the user receiver, u, to the kth satellite
Tuk≐the tropospheric delay along the line-of-sight from the user receiver, u, to the kth satellite
Nuk≐the continuous phase cycle ambiguity from the user receiver, u, to the kth satellite
xcexL1≐the L1 carrier phase wavelength, 0.1903 meters
"ugr"uk≐the pseudorange measurement error
"xgr"uk≐the carrier phase measurement error
Clock errors are mostly due to the degradation associated with Selective Availability (SA). This intentional degradation corrupts the range accuracy by values up to several tens of meters. Studies shown it reasonable to assume that the overwhelming majority of SA errors are from clock perturbations. The US Government deactivated SA on May 2, 2000, indicating that it will not be enabled again.
Ionospheric delay is caused when the GPS signal encounters the ionosphere. The carrier wave is advanced while the code phase is delayed. These effects are partially corrected for the single-frequency user by the Klobuchar ionospheric parameters broadcast in the GPS message itself. Dual frequency receivers can, for the most part, remove these effects directly.
Tropospheric delay can be up to 30 meters for low elevation satellites due to GPS signal propagation through the lower atmosphere (troposphere). There are two primary components of the tropospheric delay, dry and wet. The dry component makes up about 90% of the total delay and can be modeled well with surface pressure data. The wet component is much more difficult to model and not well correlated with surface conditions. The wet term can add as much as 2-3 meters of uncorrected error on the GPS measurements.
Ephemeris errors occur when the reported satellite position does not match the actual position. The component of these errors along the line of sight to the user is usually less than a few meters.
Multipath errors are due to local reflections of the signal near the receiver and are tracked with delay, corrupting the range and phase measurements. These effects are very sensitive to the local environment. Tall buildings are the most commonly encountered source of the reflections that cause multipath interference.
Receiver noise is comprised of thermal noise, signal and modeling quantization. These errors are usually limited to about 1 meter for pseudorange and 1 mm for carrier phase.
Since many of the above-discussed errors are common-mode for receivers that are sufficiently proximal to one another, it is possible to use measurements from one GPS receiver at a known reference location to correct the measurements of the nearby xe2x80x9cmobilexe2x80x9d receiver (a mobile receiver may be in motion or stationary; xe2x80x9cmobilexe2x80x9d is meant to indicate its usual location being unfixed with respect to the earth""s surface). At the limit, for two receivers that share the same antenna, the only residual errors that would remain are due to receiver noise.
Using GPS measurements from one or more GPS receivers to correct another GPS receiver is called differential GPS (DGPS). Every DGPS system contains three system elements: 1) a single receiver or multiple GPS receivers at known reference (fixed) locations; 2) a mobile (unfixed) receiver; and 3) a communication link between the reference receiver(s) and the mobile unit.
Local-Area Differential GPS consists of a single reference station (a GPS receiver) at a known location measuring the errors in the pseudorange and broadcasting pseudorange corrections to mobile receiver users or a data processing and storage system via a data link. Other measurements and information from the satellites may also be received by the local-area differential GPS reference station and transmitted via the data link. Such other measurements/information includes the satellite almanac, ephemeris, carrier phase, pseudodoppler, phase bias, frequency bias, clock offset, signal strength, local angles of elevation and azimuth, and others. The operating presumption is that errors observed by a mobile user are nearly identical to those observed by a nearby reference receiver. Errors typically excepted from this presumption are local phenomena such as multipath and receiver noise. In the extreme case where the location of the mobile unit and the reference station are the same, all error sources except for multipath and receiver noise cancel out.
Local-area differential GPS can reduce position errors to as little as 0.5 meters (with smoothing). However, local-area differential GPS systems suffer from a high sensitivity to the proximity of the user to the reference station. Beyond a separation of, typically, 100 kilometers the solution degrades to an unacceptable degree. As such, for functionally acceptable DGPS corrections to be available over the entire Coterminous United States (CONUS), over 500 stations are required.
The use of wide-area differential GPS for the aviation community is currently under development by the FAA and is called the Wide Area Augmentation System (WAAS). The data link employed by this system is a geostationary satellite, which has a semi-major axis of 42,000 km and a nearly zero degree inclination. The major advantage of this satellite orbit configuration is that it is synchronous with the rotation of the Earth and, therefore, is at all times in a practically fixed position relative to mobile receiver users and reference stations.
The principle behind wide-area differential GPS is the use of multiple GPS reference stations to form xe2x80x9cvectorxe2x80x9d corrections for each satellite in view of all or a subset of the GPS reference stations. The vector corrections are broken down into the components of the error sources to GPS. In a wide-area differential GPS system, the corrections include satellite ephemeris, satellite clock and the ionosphere. As in local-area differential GPS systems, multipath errors and receiver noise are not corrected, as these are purely local phenomena related exclusively to the mobile unit. The vector corrections are formed by making simultaneous measurements at multiple reference stations of the same GPS satellite observables. Observables that are recorded at the reference stations include pseudorange, pseudodoppler, carrier phase, and signal levels. Once the corrections are formulated, they are transmitted to geostationary satellites that re-broadcast the corrections to mobile users tracking the geostationary satellite. The mobile user tracks both the GPS satellites and the geostationary satellites and can thus derive pseudorange measurements not only from the GPS satellites but also the geostationary satellite. While the resultant GPS measurements, assisted by the broadcast corrections from the geostationary satellites, are more accurate, additional range sources may be used to supplement GPS. However, the vector corrections from the system are only available through the geostationary satellite. The wide-area system under development by the FAA is a xe2x80x9cclosedxe2x80x9d system meaning that the measurements are not directly available to mobile receiver users.
What is needed in the art is a system design that offers wide-area GPS corrections that can be augmented with local corrections. Such a system should enable such wide- and local-area measurements to be colocated and corresponded with a measurement taken by a mobile receiver to accurately correct the measurement taken by the mobile receiver.
In accordance with the present invention, a system and method is disclosed for corresponding data associated with primary correction of at least one first measurement of a GPS satellite characteristic to a second measurement of the GPS satellite characteristic taken at a mobile receiver.
In a preferred embodiment of the invention, a first processor is coupled to at least one wide-area GPS corrections receiver and at least one local-area GPS corrections receiver. The wide-area and local-area receivers operate to determine the first measurement and secondary data associated with correction of the determined first measurement. A second processor is optionally provided for receiving both the primary correction data from the first processor and data representing the second measurement taken at the mobile receiver.
In operation, an estimate of the location of the mobile receiver is determined by the first processor. The availability to the mobile receiver of the secondary correction data from the wide-area and local-area GPS corrections receivers is determined, preferably based on the location estimate. From the secondary correction data, primary data representing the primary correction is determined by the first processor.
If local-area secondary data is available to the mobile receiver, the primary data comprises a linear combination of all available secondary data. The linear combination comprises weighting of the local-area secondary data in proportion to distance from the mobile receiver to each local-area receiver from which the secondary data was available. If only wide-area secondary correction data is available to the mobile receiver, the primary data consists only of such wide-area data.
The primary data and the second measurement data are preferably colocated by causing the first processor to transmit the primary correction data to the mobile receiver. Alternatively, the primary data and the second measurement data are colocated by transmitting the second measurement to the first processor or optional second processor.
In further accordance with the present invention, a wide-area reference station records the wide-area system vector corrections as well as local-area differential GPS measurements. This ensemble of information along with the satellite almanac, ephemeris, and other data is transmitted to a central collection and processing center for refinement and integration with information from other wide-area and local-area reference receivers. This represents a system design that offers wide-area GPS corrections that can be augmented with local corrections. Such a system enables such wide- and local-area measurements to be colocated and corresponded with a measurement taken by a mobile receiver to accurately correct the measurement taken by the mobile receiver.